1. Technical Field
The present invention relates to optical modulators in general, and in particular to a method for fabricating butt-coupled electro-absorptive modulators.
2. Description of Related Art
Silicon-based modulators are commonly employed in photonic integrated circuits. However, the indirect bandgap of silicon can prevent any electric field induced light modulation. Thus, devices having germanium grown on silicon wafers are becoming more popular because of the relatively small difference between the indirect and direct bandgaps of germanium, which can overcome some of the difficulties stemmed from silicon-based devices.
Proper device design and fabrication are very important in forming efficient germanium-silicon based modulators because there are many challenges in making workable germanium-silicon based modulators. Ideally, germanium-silicon based electro-absorptive modulators should have the lowest possible loss at a voltage-off state with the highest possible extinction ratio at a voltage-on state. Because germanium-silicon based electro-absorptive modulators inevitably have transmission loss due to the absorption of indirect band gap, the length of germanium-silicon active region has to be very short, typically less than 150 μm, in order to achieve low insertion loss at the voltage-off state. Therefore, it has to be coupled to a low-loss waveguide, such as a silicon waveguide, for on-chip applications. Since germanium-silicon based material system has a relatively high index contrast and its single mode dimension is very small (the single mode cut-off dimension is less than 1 μm), it is a big challenge to achieve an efficient waveguide-modulator coupling. Another problem is that standard reactive ion etching (RIE) of germanium-silicon based material usually results in rough sidewalls that increases the scattering loss in the germanium-silicon based electro-absorptive modulator.
Consequently, it would be desirable to provide an improved method for fabricating germanium-silicon based electro-absorptive modulators.